It is common commercial practice world-wide to produce gasoline, heating oil and diesel fuel by cracking heavier petroleum fractions to lighter, more valuable materials. One of the major commercial techniques for accomplishing this conversion is through fluid catalytic cracking or FCC. In FCC a fraction of petroleum feed such as vacuum gas oil, is contacted with particles of hot, reactive catalyst at high temperatures and medium pressures of about 1 to 4 atmospheres. The catalyst should be in adequate quantity and at high temperature to vaporize the feed, raise the oil feed to a cracking temperature of about 480° C. to 590° C. and supply the endothermic heat of reaction. The oil and catalyst flow together for hydrocarbon conversion to occur. During FCC conversion process of the heavy petroleum feed and middle distillate fractions, by-product coke (carbon) is deposited on the catalyst particles thereby deactivating the catalyst. The carbon deposited catalyst particles are separated from cracked petroleum product in the cyclone. The product is recovered in fractionators and coked catalyst is transported to regenerator. The hot regenerated catalyst is returned to riser for further reaction thus completing the cycle. The FCC process is heat balanced. Burning of coke in the regenerator provides heat, to provide all the heat requirements of the cracking systems. There is a firm liaison between the amount of coke produced during cracking, coke burnt off during regeneration and the heated catalyst returns to the cracking side of the process. This combination is not totally independent and controllable. It is in turn partly influenced by the nature of the petroleum fraction to make more or less coke under a given set of cracking conditions.
It has been the usual FCC operational practice to work out a balance of all the effects and counter effects to adjust feeds, residence times, etc., to achieve a heat balanced operation. Hence the type of feed, feed rate, feed temperature, type of catalyst, catalyst to oil ratio, contact time, reaction temperature etc., are adjusted on the cracking side so as to produce a desirable product slate while depositing ample amount of coke on the catalyst to satisfy the system.
Changes in the system, such as the inherent coking tendency of the feed or catalyst, (over which refiner has no control), are managed with heat exchangers. Airflow rates and/feed pre-heaters are used to adjust the operations to the peculiar requirements of particular situations.
There has always been a need on refiners, to reduce emissions of carbon monoxide and nitrogen oxides like NO, N2O (NOx) from FCC regenerator in the flue gas. This has in some cases, been accomplished through the use of carbon monoxide boilers and scrubbers. Though these work well, but require sizeable capital investment and pose a problem with respect to maintenance and repair. Thus when a separate on stream carbon monoxide oxidation system is taken out off line for routine or emergency repair and maintenance, there is an inherent increase in the carbon monoxide and NOx emission from the FCC regenerator. As a result to maintain the purity of stack gas emission standards within tolerable limits, it has been considered necessary to have backup carbon monoxide and NOx control systems; or to modify the operation of the whole FCC; or to vary from emission control requirements.
Substantial progress has been made towards modifying the operation of an FCC process so as to reduce carbon monoxide and NOx in the regenerator off gas by reducing or eliminating the need for downstream carbon monoxide oxidation facilities. This is being accomplished by either increasing the air feed to the regenerator or by the use of multifunctional catalyst additive. Burning carbon monoxide in the regenerator tends to increase the heat in the regenerator. This has some beneficial effects upon some FCC operations in that it reduces residual carbon on regenerated catalyst, it may permit a reduction in catalyst inventory; and/or a lower catalyst to oil ratio; and/or a higher cracking temperature. It may also permit cracking feed stocks which are inherently low coke makers because burning less coke substantially all the way to carbon dioxide may generate sufficient heat to make up for the smaller amount of coke.
FCC processes emit carbon monoxide, NOx etc. in regenerator. As a result, there is an environmental regulation on refiners, to reduce emissions of carbon monoxide and NOx from FCC regenerator flue gas and also to dispose the used catalysts safely. Reduction of carbon monoxide in some cases has been achieved by the use of incinerators or carbon monoxide boilers. Nitrogen oxides reduction has been achieved by injecting ammonia. The use of incinerators or boilers involves substantial capital investment. Carbon monoxide boilers also create problems with respect to maintenance and repair. Whenever carbon monoxide oxidation system is taken out for various reasons, there is increase in carbon monoxide emissions from the FCC regenerator. The disposal of used catalysts is done in many ways like down grading the material and using for other applications or the material is sent for metal recovery and land filling.
Therefore, in order to maintain the emission of harmful gases in stack gas within allowable limits, it has been thought necessary to have support carbon monoxide control systems or modify the operation of the whole FCC by use of an external agent to reduce emissions of carbon monoxide and NOx from FCC regenerator flue gas. Effective utilization of the heat generated during the carbon monoxide oxidation process is also required.
U.S. Pat. No. 4,064,039 discloses a catalyst containing Platinum (Pt) or Rhenium (Re) which improves carbon monoxide burning in regenerator dense bed. This patent describes the use of modified FCC catalyst with a noble metal. However this patent does not discuss about the use of refinery discarded catalyst.
U.S. Pat. Nos. 4,915,035 and 4,915,037 teach the use of low surface area porous support (having surface area greater than 50 m2/g) containing silica, alumina, silica/alumina, Kaolin and mixtures containing Pt, Rhodium, Osmium for improving carbon monoxide oxidation. The average particle diameter is in the range of 400-1200 micron. These patents disclose the concentration of Pt in the range of 0.01 to 100 ppm but other metals like Rh, Palladium (Pd) and Osmium (Os) are also disclosed. Actual supports or support material's composition is not revealed in these documents. Besides this the particle size disclosed in these documents is on higher side which may create no uniformity in fluidization in reactor and regenerator. The porous support disclosed in both these documents is used for fluid bed boiler. These patents do not use the refinery discarded material and the end use is different.
U.S. Pat. No. 4,350,615 teaches that small amounts of promoter comprising Pd and Ruthenium (Ru) enhance the oxidation of carbon monoxide without substantially increasing NOx emissions. The Pd—Ru promoter also enhances the capture of sulfur oxides by using suitable adsorbents within the regeneration zone of FCC without causing excess amounts of nitrogen oxides. This patent does not talk about the recycling of spent or discarded material from refinery.
U.S. Pat. No. 4,544,645 discloses a process for cracking a sulfur containing hydrocarbon and an improved oxidation promoter for converting SO2 to SO3 comprising an intimate association of Pd and at least one other metal selected from the group consisting Pd, Osmium (Os), Iridium (Ir), Re and Rh. This document does not disclose carbon monoxide and NOx reactions.
U.S. Pat. No. 4,300,997 teaches that small amount of Pd and Ru enhances the conversion of carbon monoxide with out affecting the NOx levels in the FCC regenerator. Pd to Ru metal ratio varies from 0.1 to 10 weight percentage. This also has additional advantage of removal of SOx without formation of excessive NOx. However the invention did not use the refinery discarded material.
U.S. Pat. No. 6,902,665 describes compositions comprising a component containing an acidic support, an alkali metal and or/alkaline earth metal, transition metal oxide having oxygen storage capability and Pd metal. The acidic oxide support preferably contains silica alumina. This document describes that certain class of compositions are very effective for CO oxidation promotion, while minimizing the NOx. In this document Ceria is essentially used as integral part of support with the concentration of 2 to 50 parts by weight percentage. However this invention did not use the refinery discarded material.
U.S. Pat. No. 4,608,357 discloses novel catalyst formulations for promoting CO oxidation. This document mainly contains Pd and at least one other precious metal on a porous silica alumina support obtained by leaching sufficient silica from shaped particles of calcined clay with caustic solution to impart porosity to the particles. The support particles preferably has Al2O3/SiO2 molar ratio in the range of 1.58 to 1.64; total pore volume of 0.15 cc/g′ surface area in the range of 20-60 m2/g; pore structure in the range of 100-600 Å; pore diameter in the range of 150 Å to 350 Å; the noble metal Pd in the range of 50-600 ppm and precious metal Re in the range 300 to 500 ppm. Subject patent deals with only CO oxidation activity but not with NO reduction. The patent also did not use the refinery discarded material.
U.S. Pat. No. 7,045,056 describes a composition for controlling CO and NOx emissions during FCC processes comprising, acidic oxide support, cerium oxide, lanthanide oxide other than Ceria such as praseodymium oxide, optionally oxide of a metal from groups Ib, IIb such as copper, silver and zinc and Pt or Pd. Different formulations are prepared using Al2O3, copper modified Al2O3, Ceria modified alumina. Samples prepared were deactivated by steaming at 1500 F for 4 Hrs using 100% steam. Noble metals Pt and Pd mixture from 50 to 1500 ppm; feed concentration of 0.1 wt % of nitrogen is used. At least one oxide of lanthanide series element viz cerium, praseodymium oxide is used in this document. Even though use of rare earth metals increases the efficiency of the process, however the invention did not use the refinery discarded material.
Not withstanding the amount of material available in the prior art, there is still need for recycling the discarded refinery catalyst material or use fresh superior support for making new multifunctional catalyst additives with better physical properties for CO and NOx reduction without having dilution effect.